Astronomy

Astronomy and Astrophysics

The following is a metanode of topics, terms,
concepts, and ideas related to the fields of
astronomy and astrophysics. It is by no
means complete, but its organization will
hopefully give the casual reader an idea of
what the science of astronomy entails.
The nodes are outlined in similar order to
what one might find in a college-level,
introductory textbook on astronomy, beginning
with the most basic phenomenology of astronomy
(what is visible to the naked eye) and a
description of the tools and fields of
observational astronomy. From there, we travel
outwards from the Earth to cover the
solar system, the stars, the Milky WayGalaxy, other galaxies and extragalactic
astronomy, and finally cosmology. We have
combined "astronomy" (the observational science)
with "astrophysics" in a single node, since
there is substantial crossover between the two
in modern times, and "astronomy" is a logical
place to look for astrophysical information.

Note that a metanode of famous astronomers
exists separately, under astronomer.

Also note that planetary science is
given very short shrift here. This is
partly because it is still distinct from
classical astronomy and astrophysics -- it is
the only field of astronomy where we have
actually visited our objects of study -- and
partly because it encompasses elements of
geology, chemistry, meteorology, and
biology. There is currently little organized
information about planetary science on e2; as
more writeups on this field are added, we may
consider making a separate index. Noders
familiar with planetary science are urged
to contact E2_Science if they are interested
in organizing this.

This index is incomplete and will be for some time,
so please contribute! Dead links will be noted with
a (*) after the node title -- if you see one, please
fill it! If you would like to have a writeup or
topic added to this node, please /msg E2_Science.

Observational Astronomy

Astronomy is one of the oldest sciences, primarily
because the apparent motions of objects in the
heavens relate to one of our most fundamental
concepts: the passage of time. Nearly every living
thing on Earth is affected by celestial cycles,
especially the diurnal motion of the Sun, the tides
of the moon, and the seasons of the year. So it is
unsurprising that our distant ancestors were curious
about the sky. Furthermore, since it was (and still
largely is) utterly remote from us, "those lights
in the sky" produced endless speculations on their
nature, and their role in our lives.

Most of the fundamentals of astronomy relate to
three things: position and time, both of which are
deeply linked, and brightness. To the human eye,
most stars appeared to lie on a giant sphere --
The Celestial Sphere -- which rotated endlessly
about our world, changing our night sky during the
course of the year. Most human cultures picked
out patterns on this celestial sphere (since pattern
recognition is one thing humans are especially good
at), and grouped these stars into constellations.
The apparent motions of the Sun and planets were
superimposed on this sphere, with the Sun's daily
and yearly motions, and the moon's monthly
apparitions having special importance. Thus we
began to understand and measure such fundamental
concepts as solstice and equinox, ecliptic
(and of course, the Zodiac), analemma,
sidereal period and synodic period, and so on.
Later, as our understanding of the heavens became
more extensive and sophisticated, we began to
quantify things like time and position, giving
rise to concepts like equatorial coordinates.
And of course it was known to the ancients that
stars had different apparent brightnesses, leading
again to a desire to quantify this, as with magnitude.

The links listed below will lead you to several
fundamental concepts in astronomy. Reading and
understanding each will give you a good idea of how
we understand the heavens to be organized, but
will also give you a glimpse of what our remote
ancestors were capable of understanding about the
universe, without benefit of telescope or computer.

Along with instrumentation to view the heavens,
we began to develop means for permanently storing
or otherwise quantifying our observations. Beginning
with the early photographic plates of the early
19th century, we have since developed technology for
performing photometry and spectroscopy of
celestial objects. The links below list a few of
the important ones.

Fields of Astronomy

With the incredible advances of both technology
and knowledge of physics since the early 17th
century have come similar advances in astronomical
technology and understanding. We now know that
visible light is but a small portion of the energy
emitted by astronomical objects in the universe, and
that observing different wavelengths of light can
provide a wealth of physical information about objects
in the universe. The first hints that there was more
to the universe than meets the eye came when
William Herschel placed thermometers in sunlight
dispersed by prisms, and found temperature changes
even when the thermometer lay beyond the reddest
light visible. Now, we know astronomical objects
can emit light from very low frequency
radio waves to gamma rays so energetic
that each photon has the energy of a speeding bullet.

Below are links to the various fields of astronomy:
most of these fields are considered distinct, since
each requires its own unique forms of telescope
and detectors. Some even require satellites in space.
In addition to the formal research fields, we also
mention the work of amateurs in Amateur Astronomy
and Astrophotography. The work of amateur
astronomers and astronomy enthusiasts has always
been an important part of scientific astronomy, from
the discovery of variable stars and
early advances in radio astronomy to
rapid observations of transient events like
gamma-ray bursts. The line between "amateur"
astronomers and professionals has always been
blurry, and is growing ever more so.

Famous Observatories

Though one can easily conduct astronomy from
one's backyard or rooftop, most societies have
created special places for conducting observations
of the heavens. Since ancient times, many cultures
consider astronomical observations sacred and vital
to society, placing great importance in the
construction of observatories. In modern times,
scientific observatories are placed in increasingly
remote locations out of necessity, as cities and
towns become ever more light-polluted and crowded.
And some forms of astronomy require
observations in space, necessitating orbiting
observatories and even robotic explorers of our
solar system. Below is an incomplete list of famous
observatories and satellites in the history of
astronomy, many of which are still in existence today.

The Solar System and Planetary Science

The solar system is comprised of everything
contained within the gravitational and
heliospheric influence of The Sun. It
consists of the Sun itself, the nine major
planets and their moons, the
asteroids, the comets, dust, gas,
and everything else contained within a sphere of
(roughly) one light-year in radius about the Sun.

The Sun, the closest star to Earth, is
the most important part of our solar system.
Because it is the most massive object in the
solar system, it acts as the gravitational
focus of all other matter in the solar system.
It also provides essentially all of the
luminous energy of the solar system, and all
of the planets and other objects shine by
reflecting light from the Sun to our eyes. The Sun
also forms the basis for our understanding of
all other stars in the universe, and it is by
detailed study of the Sun that we began to
understand the inner workings of stars.
The Sun also provided some early indications
that not all things in the heavens were truly
"perfect". For example, the discovery of
sunspots showed that the surface of
the Sun was not unblemished, and soon other
examples of "solar activity" were discovered.

The planets are also an important part of our
solar system. The planets are important
from a physical standpoint -- for example,
while the Sun contains most of the mass and
emits most of the luminous energy in our solar
system, the orbiting planets carry over 99
percent of the angular momentum of the solar
system. And obviously, the planets (at least
a few of them) are important as known or
potential abodes for life. However, the planets
are also important from a philosophical
standpoint. They showed our early ancestors
the "stars" were not "fixed" in space, and could
indeed wander about the Celestial Sphere. From
this, they obtained their name planet
from the Greek planetes, meaning "wanderers".
Eventually, study of the planets led to a
deeper understanding of how the universe works,
though an understanding of gravitation.

And of course, other members of our solar
system continue to have profound scientific
and cultural impacts. For example,
comets have been considered important
phenomena -- both good and
bad -- since ancient
times, and they remain an important cultural
and artistic influence today. The discovery
of asteroids beginning in the 19th
century continues to be an important scientific
task, and we now know this task may have
profound implications for
human civilization.

Stellar Astronomy and Astrophysics

Stellar Astronomy and Astrophysics is
an important (and very broad) topic.
The Stars may rightly be considered the
most important objects in the universe. After
the big bang, it was the stars that generated
all of the atoms of all of the elements more
complex than helium (including most of the
atoms that make up you). These elements
were distributed in supernova explosions and
the dusty winds of dying stars. And it
is the stars that generate most of the visible light
in the universe; when you look at distant galaxies,
most of the light you see isn't the "galaxy" itself,
but the stars it is made of. Because stars are
so fundamentally important, it is worth spending
more time reading about them.

Stars are like giant and complex physical
laboratories, and we first spend some time
talking about the physics of stellar astronomy.
Below are listed several nodes on physics relevant
to stellar astronomy, including: a primer on
electromagnetic radiation, and nodes on
radiative transfer and various emission processes
important in stars; nodes on the behavior of
the gas that makes up the stars, and how it
behaves; and discussions of fundamental physics
important in stars, like gravitation and nuclear
physics. While some of these topics are somewhat
advanced, browsing them should give you an idea
of how complex the stars are.

After the preliminaries, we discuss what we know
about the stars, and how we know it. Most
important of all of these nodes are the
discussions of stellar classification in
stellar spectra and Hertzsprung-Russell diagram.
These two topics summarize best what we know
about stars, and why. From there, we spend time
talking about the details of stars and stellar
populations, as in Populations I,
II, and III stars,
the various evolutionary stages of stars, and
finally, the very diverse field of
variable stars and variable
star research. We conclude with nodes about
some of the most extreme stars known:
neutron stars,
black holes, and their associated
phenomena -- including the (no longer so)
mysterious gamma ray bursts.

From the stars, we move on to nodes about
galaxies. Galaxies are huge
aggregations of matter that formed from
faint ripples of density in
the early universe. Since
that time, the matter in galaxies has slowly
collected into stars and other types of
objects that make up the "modern universe",
including vast clouds of
gas and dust, clusters
of stars, and even more exotic things like
emission line nebulae and supermassive black
holes.

First we begin with Galaxy, Milky Way,
and The Great Debate, nodes which explain
what galaxies are, and how we know this. In particular, The Great Debate is an important node to read.
The fact that the "spiral nebulae" are
"island universes" separate from our own Milky
Way Galaxy is a relatively recent discovery in
astronomy, and the history of this
discovery in an important one.

From there we move on to talk about the building
blocks of galaxies -- the star clusters, the nebulae,
and the interstellar medium -- in some detail,
before finally discussing the different kinds of
galaxies, and their classification systems.
Finally, we end with discussions of the physics of
galaxies, the nature of active galaxies and
supermassive black holes, and finally the greatest
building blocks of the universe -- the galaxy clusters.

Finally, we end this metanode with a discussion
of cosmology -- the study of the universe as
a whole. Cosmology is perhaps the oldest of
intellectual pursuits, as two of its fundamental
questions are how was the universe created?,
and why is it the way it is?. As scientific
knowledge developed over time, our understanding of
the universe expanded as well. Today, the general
scientific consensus is that the
universe is a "relic" of a great event -- the big bang --
which created all of the great structures we see in the
universe today. The universe is filled with
billions and billions of galaxies, some beyond the
reach of even our largest telescopes. These galaxies
are gathered together into great clusters and
sheets, surrounding vast, empty voids of space. The
redshifts of external galaxies are believed (again, by
general consensus) to be due to the expansion of
space itself -- another result of the big bang.
Cosmology entails the study of these great structures
and the underlying physics of matter and energy
to explain how the universe came to be and why it looks
the way it does. One of the fascinating things about
cosmology is that it merges studies of the greatest
structures in the universe with the most
infinitesimal particles and fundamental physics.
We do not yet know how the universe was created or
how it will end, because we do not yet fully
understand the most
basic physics of matter and energy.

Below are a few links covering this very broad
topic, beginning with a discussion of how
scientists believe the universe began and why. This
is followed by nodes on how we measure
distances in astronomy. This is a very
important topic in cosmology; the universe appears
very two-dimensional to our eyes, so it isn't
easy to estimate how far away things are. Finally,
we end with a discussion of some of the
physics of cosmology, including the
expanding universe, the newly-resurrected concept
of the cosmological constant, and the ultimate
fate of... everything.

In the classicalLiberal Arts, the ancients considered music the extension of mathematics in time, and geometry the extension of number in space. Observing the clockwork motions of the moon, planets, and other heavenly bodies, they synthesized mathematical notions in space and time into astronomy, or "the music of the spheres". This colorful attribution is often credited to Pythagoras, although the first reference in literature appears near the end of Plato's Republic. Since the classical age, physics has informed astronomy so as to form a new field, astrophysics.

Astronomy is also a song by Blue Öyster Cult, first appearing on the 1974 Secret Treaties release, and reworked (in my opinion, much improved, if shorter), for the 1988 Imaginos release. Metallica also covered BÖC's Astronomy on their 1998 Garage Inc. album. BÖC fans might wonder why Burnin' for You and Fear the Reaper get so much more airplay; Astronomy is critically acclaimed, but lacks a mainstream pop theme.

Perhaps the oldest of the sciences, astronomy originated as simple observation and recording of regular celestial movements. Ancient Greekstudents of astronomy understood that the Sun was the center of the solar system, made reasonable estimates for the sizes and distances of the Sun and the Moon, and made a close estimate of the size of the Earth.

Not from the stars do I my judgment pluck;
And yet methinks I have astronomy.
Shak.

2.

The science which treats of the celestial bodies, of their magnitudes, motions, distances, periods of revolution, eclipses, constitution, physical condition, and of the causes of their various phenomena.